The long-term objective of the proposed experiments is to understand the genetic and molecular mechanisms of pattern formation in multicellular organisms. The organism chosen for laboratory experiments is the flowering plant Arabidopsis thaliana, and the patterns to be studied are the patterns of organ identity and differentiated cell type in flowers. The choice of flowers and of Arabidopsis is for experimental convenience and speed: the plant is small, grows quickly, has powerful genetics, and is well-suited for facile molecular cloning and transformation. Flowers are to be used because they are highly patterned and demonstrate the key features of developmental pattern formation in all organisms, yet contain very few cell types, develop rapidly, and are inessential for plant viability. Mutants with abnormal flowers can therefore be easily obtained, grown, and studied. The health relevance of causal analyses of pattern formation is to birth defects, in which patterns have formed incorrectly; to inherited developmental disorders, in which regulatory genes such as those emphasized in this proposal act aberrantly; and to tumors, which consist of cells that have escaped normal negative controls on cell division, cellular differentiation, and cellular migration. The specific experiments proposed are to understand the regulation and activities of the floral organ-identity and regulatory gene AGAMOUS. AGAMOUS plays three key roles in floral pattern formation: it is necessary for specifying the stamens and carpels, it is necessary and sufficient to negatively regulate the genes that specify sepals and petals, and it is required for appropriate cell division patterns in the center of developing flower primordia. The protein product of the AGAMOUS gene is a DNA-binding transcriptional activator that is related at the level of amino acid sequence to human transcriptional regulators such as serum response factor, which regulates the c-fos oncogene, and MEF-2, which regulates muscle development. Three sets of experiments are proposed. The first are to understand the spatially-specific expression of AGAMOUS. This is in large part explained by two negative regulators of AGAMOUS that have been found. The genes encoding these negative regulators will be studied and cloned. Second, the activity of AGAMOUS in specifying organ identity will be studied. New methods have been developed and will be applied to finding the downstream genes activated by the A GA MO US gene product. Finally, the role of cell-cell communication in developing flowers will be studied by making genetic mosaics, again using new methods that have been devised for this type of experiment.